The invention relates generally to arrangements for the treatment of particulate matter wherein the particulate matter is entrained by a gas for treatment. Of particular interest to the invention are arrangements for the drying of particulate matter, that is, pneumatic conveying dryers.
Pneumatic conveying dryers have already for a long time been used for the drying, and also for the heating, of finely divided, moist particulate materials. Generally, a pneumatic conveying dryer includes a vertically arranged conduit through which a hot gas flows from bottom to top with a relatively high velocity. The material to be dried is fed in at a bottom portion of the conduit, entrained by the gas stream and dried while being carried upwardly through the conduit by the gas stream. The velocity of flow of the hot gas must here lie well above that velocity at which the coarse particles of the particulate material would remain suspended. The drying occurs primarily in the lower and middle regions of the conduit. As a rule, the upper region of the conduit opens into a cyclone which serves for separating the hot particulate material from the gas stream.
It is known to construct such pneumatic conveying dryers with a single stage, that is, with a single conduit for drying. It is further known, however, to construct such pneumatic conveying dryers with two or more stages. Two-stage pneumatic conveying dryers are, for instance, utilized for the thermal treatment of wet coking coal. Here, throughputs of 50 tons per hour have been achieved heretofore.
The gas inlet end of a given vertically arranged drying conduit is generally constructed in the form of a 90.degree. or a 180.degree. elbow. Thus, the hot gas is produced in a horizontally oriented combustion chamber. The gas produced in the combustion chamber must then be conveyed into the single drying conduit constituting the sole stage of a single-stage dryer or, for the case of a two-stage dryer, for instance, into the drying conduit constituting the second stage. Here, the gas inlet end of the drying conduit will be in the form of a 90.degree. elbow. On the other hand, for the case of a two-stage dryer, for example, the gas obtained from the cyclone of the second stage is conveyed vertically downwardly to the entrance of the first stage. Here, a 180.degree. elbow is required in order that the gas obtained from the cyclone of the second stage may enter the drying conduit constituting the first stage.
The introduction of the particulate material into the lower region of the vertical conduit occurs immediately downstream of the elbow and, for the case of moist materials, the feeding of the particulate material has heretofore been accomplished by free fall or by means of rotating impellers. The latter are particularly advantageously used when the particulate material to be fed in is moist and consists of particles which tend to adhere to one another. For the feeding of dry, readily fluidized particulate materials, it has also become known to use vibrating beds of particulate material which surround the cross-section of the conduit in the form of an annulus. The latter type of particulate material is also satisfactorily blown into the drying conduit by means of gas streams.
A disadvantage of the prior art resides in that the known arrangements for the feeding of moist and difficult-to-fluidize particulate materials can be used only when the diameter of the drying conduit is one meter or less. The reason is that the finely divided particulate material can be uniformly distributed over the entire conduit cross-section during feeding only when the conduit diameter is small. A uniform distribution of the particulate material over the conduit cross-section is, however, of importance since it is only with such a uniform distribution that a homogeneous transport and drying of the material along its path of travel can be insured.
To illustrate one of the considerations involved, it is pointed out that for smaller apparatus having, for example, a throughput capacity of 10 tons per hour, the requisite conduit diameter is of the order of 300 to 400 millimeters so that a uniform distribution of the material poses no difficulties. However, as just indicated, this is not the case for conduit diameters exceeding one meter. Since the maximum throughput capacity achievable for conduit diameters of less than one meter is 50 tons per hour, it will be appreciated that the prior art feeding means poses a rather severe restriction on the throughput capacities which may be obtained.